Putting AT&T’s VoLTE Service to the Test

Signals Research Group (SRG) worked with Spirent Communications recently to complete a benchmark study of AT&T’s VoLTE service offering in Minneapolis-St. Paul. For those of you keeping score at home, this endeavor marks the umpteenth time that SRG and Spirent have collaborated together over the last decade. For SRG and its readership, the benefits are huge since these collaborative studies provide great insight into how 3GPP technologies, in general, as well as individual vendor implementations of these technologies, perform. No marketing B.S., “just the facts, Ma’am.”

The three of us – Mike Thelander (SRG), Emil Olbrich (SRG) and Kurt Bantle (Spirent) – spent three days in a cramped car driving around the Twin Cities and sitting in an IKEA parking lot near the Mall of America, Minneapolis-St. Paul International Airport, and a major freeway, while collecting performance data on how VoLTE, Skype and 3G CS voice compare. SRG has since analyzed the data and published the results in its subscription-based Signals Ahead research product.

In addition to discovering a greater appreciation for fried Walleye that has been breaded in Ritz crackers [you had to be there] we also established greater respect for the true benefits of VoLTE, for both operators and consumers. With Nomad we observed a significant difference in the call quality of VoLTE versus 3G CS voice (NB-AMR) and a measurable difference with Skype. Obviously the call setup times with VoLTE were also nearly twice as fast as CSFB. With network loading, and in particular with background applications transferring data on the same smartphone, the differences in the call quality between VoLTE and Skype increased considerably. Since Skype doesn’t support QCI=1 it was easy to bring the application to its knees with Nomad frequently unable to detect the presence of voice activity.

Consumers should also appreciate the measurable differences in current drain and power consumption between VoLTE and Skype with the latter likely to drain the battery 35% faster than VoLTE. VoLTE’s advantage stems in part from the lower codec rate used in the AT&T network (only 12.6 kbps), but as previously noted VoLTE still delivered higher MOS values. Additionally, VoLTE took greater advantage of DTX/DRX (Discontinuous Transmission / Reception) and in the longer term it will be able to take advantage of short DTX/DRX and SPS (Semi-Persistent Scheduling), which are features that are only supported with a QCI=1 application. With these features the battery life during a VoLTE call should exceed that of 3G CS voice – presently the latter has a 15-20% advantage.

From an operator’s perspective, they’ll appreciate the relative efficiency of VoLTE compared with Skype, and presumably other OTT voice applications. We observed a ~ten-fold difference in the amount of network resources required to support a VoLTE call versus a Skype Voice call. Although part of the difference is due to the low WB-AMR codec rate used in the AT&T network, it doesn’t come close to explaining the differences. RoHC (Robust Header Compression) definitely played a leading role according to the data that we analyzed while as previously noted the higher codec rate used by Skype didn’t result in a higher call quality.

SRG is now working with Spirent on part two of its study on VoLTE. Although we are still working to nail down the particulars, it will involve a fair amount of lab testing in which we recreate scenarios that we tested in the field, albeit in a controlled lab setting, and most likely additional OTT voice applications and VoLTE clients.

If you are interested in reading more about the recent study, click here to download the report preview.